AsynchDNS errors are not handled correctly in multi in 7.65

[ngg]

This issue is split off from #4135 as I think it is a distinct issue (probably caused by the same underlying bug).

Since #3651 has been merged, errors during asynchronous IP resolution are not handled correctly when using the multi interface, resulting in hanging connections.

In the following example, we have two transfers to the same non-existing host, and the multi is configured to run maximum 1 connection parallel, this way the second transfer tries to reuse the first connection and hangs. Before #3651 the first connection was removed from the connection cache after an error, and the second transfer created a new connection. After that patch, somehow the first connection stays in the cache and the second transfer waits for it indefinitely because it thinks "Connection #0 is still name resolving".

If I disable the threaded resolver, it works ok.

#include <curl/curl.h>
#include <stdio.h>

static int debug_callback(CURL* handle, curl_infotype type, char* data, size_t size, void* userptr)
{
    switch (type) {
    case CURLINFO_TEXT:
        fprintf(stderr, "== Info (%d): %s", (int)userptr, data);
        break;
    }
    return 0;
}

int main(void)
{
    curl_global_init(CURL_GLOBAL_DEFAULT);

    CURLM* multi_handle = curl_multi_init();
    curl_multi_setopt(multi_handle, CURLMOPT_MAX_TOTAL_CONNECTIONS, 1L);

    for (int i = 0; i < 2; i++) {
        CURL* http_handle = curl_easy_init();
        curl_easy_setopt(http_handle, CURLOPT_URL, "http://non-existing-host.haxx.se");
        curl_easy_setopt(http_handle, CURLOPT_HTTP_VERSION, CURL_HTTP_VERSION_1_1);
        curl_easy_setopt(http_handle, CURLOPT_DEBUGFUNCTION, debug_callback);
        curl_easy_setopt(http_handle, CURLOPT_DEBUGDATA, (void*)i);
        curl_easy_setopt(http_handle, CURLOPT_VERBOSE, 1L);
        curl_multi_add_handle(multi_handle, http_handle);
    }

    int still_running, last = 0;
    do {
        CURLMcode mc = curl_multi_perform(multi_handle, &still_running);
        if (mc != CURLM_OK) {
            fprintf(stderr, "curl_multi_perform() failed, code %d.\n", mc);
            return 1;
        }
        if (still_running != last) {
            fprintf(stderr, "still_running = %d\n", still_running);
            last = still_running;
        }

        int numfds;
        mc = curl_multi_wait(multi_handle, NULL, 0, 1000, &numfds);
        if (mc != CURLM_OK) {
            fprintf(stderr, "curl_multi_wait() failed, code %d.\n", mc);
            return 1;
        }
    } while (still_running);
}

Current output (it hangs in that state):

== Info (0): STATE: INIT => CONNECT handle 0x5627fd09ea98; line 1348 (connection #-5000)
== Info (0): Added connection 0. The cache now contains 1 members
== Info (0): STATE: CONNECT => WAITRESOLVE handle 0x5627fd09ea98; line 1384 (connection #0)
== Info (1): STATE: INIT => CONNECT handle 0x5627fd0a4348; line 1348 (connection #-5000)
== Info (1): Found bundle for host non-existing-host.haxx.se: 0x5627fd0753f8 [serially]
== Info (1): No connections available in cache
== Info (1): No connections available.
== Info (1): STATE: CONNECT => CONNECT_PEND handle 0x5627fd0a4348; line 1372 (connection #-5000)
still_running = 2
== Info (0): Could not resolve host: non-existing-host.haxx.se
== Info (1): STATE: CONNECT_PEND => CONNECT handle 0x5627fd0a4348; line 2999 (connection #-5000)
== Info (0): Curl_disconnect when inuse: 1
== Info (0): Expire cleared (transfer 0x5627fd09ea98)
== Info (1): Found bundle for host non-existing-host.haxx.se: 0x5627fd0753f8 [serially]
== Info (1): Connection #0 is still name resolving, can't reuse
== Info (1): No connections available in cache
== Info (1): No connections available.
== Info (1): STATE: CONNECT => CONNECT_PEND handle 0x5627fd0a4348; line 1372 (connection #-5000)
still_running = 1

Good output from older curl (and the program exits):

== Info (0): STATE: INIT => CONNECT handle 0x55cb42ff4af8; line 1361 (connection #-5000)
== Info (0): Added connection 0. The cache now contains 1 members
== Info (0): STATE: CONNECT => WAITRESOLVE handle 0x55cb42ff4af8; line 1402 (connection #0)
== Info (1): STATE: INIT => CONNECT handle 0x55cb42ffa3a8; line 1361 (connection #-5000)
== Info (1): Found bundle for host non-existing-host.haxx.se: 0x55cb42fcb3f8 [serially]
== Info (1): Server doesn't support multi-use (yet)
== Info (1): No connections available in cache
== Info (1): No connections available.
== Info (1): STATE: CONNECT => CONNECT_PEND handle 0x55cb42ffa3a8; line 1385 (connection #-5000)
still_running = 2
== Info (0): Could not resolve host: non-existing-host.haxx.se
== Info (1): STATE: CONNECT_PEND => CONNECT handle 0x55cb42ffa3a8; line 3111 (connection #-5000)
== Info (0): Closing connection 0
== Info (0): The cache now contains 0 members
== Info (0): Expire cleared (transfer 0x55cb42ff4af8)
== Info (1): Added connection 1. The cache now contains 1 members
== Info (1): STATE: CONNECT => WAITRESOLVE handle 0x55cb42ffa3a8; line 1402 (connection #1)
still_running = 1
== Info (1): Could not resolve host: non-existing-host.haxx.se
== Info (1): Closing connection 1
== Info (1): The cache now contains 0 members
== Info (1): Expire cleared (transfer 0x55cb42ffa3a8)
still_running = 0

[andreibica]

I'm having the same issue. I've reproduce it using a modified multi-uv.c example: just set the CURLMOPT_MAX_TOTAL_CONNECTIONS to 1 and run the example with a set of invalid domains.

[ngg]

So the problem is that multi_runsingle detects a stream_error and calls Curl_disconnect and then detach_connection. The former does not do anything since CONN_INUSE(conn) is true because conn->easyq still contains data (which will be only later removed in detach_connection). If I switch the order of these two calls, it seems to work, though I have no idea if it would cause any other trouble, or if there are other places where something like this should be called before Curl_disconnect.